"1 out of 100" is not a ratio, but a proportion or a fraction, and these can increase and decrease in the English language as they can in mathematics.Is the second sentence below referring to an increase or decrease of the ratio?
"In 2010, the ratio was 1 out of 100. In 2013, the ratio became 1 out of 200."
Sorry, I did not mean to be overly critical; you will find that many English speakers use "ratio" in exactly the way you did, and no one really worries about it. Your original sentence was grammatically correct, understandable and unambiguous. And yes, you can use "decrease".I did not know that the word “ratio” should not be used in instances where one quantity is said to be “out of” another quantity.
While you might use "was reduced", this is the passive voice, which might not be what you mean. Unless you specifically want to state that an unnamed agent reduced the proportion, it would be more common to say "the proportion reduced". If you told us what the sentence described, we could say whether the active or passive voice is more appropriate."In 2010, the proportion was 1 out of 100. In 2013, the proportion was reduced to 1 out of 200."